1. Field of the Invention
The present invention relates to dual-energy x-ray absorptiometry for tissue properties; and, in particular to techniques for deriving bone structure, bone strength, and risk of injury, including risk of fracture, from multiple projection dual-energy x-ray absorptiometry images.
2. Description of the Related Art
The system of bones and skeletal muscles provides structure to a human or animal body, and provides the capability to carry out activities. Bone provides the basic structural integrity of the body that carries forces and furnishes a framework for muscle.
Experience with bed rest subjects, astronauts and cosmonauts indicates that the magnitudes and patterns of bone tissue loss are extremely variable from one individual to the next, and also between different body regions. Little mass appears to be lost from the upper extremities during weightlessness; whereas the rate of mass loss from the vertebrae, pelvis, and proximal femurs of astronauts average between 1 percent and 1.6 percent per month. The rate of mass loss from those sites in postmenopausal woman average between 0.8 percent and 1.3 percent per yearxe2x80x94a substantially lower rate of loss.
Recent evidence shows that there are important differences between the ways that bone is lost in aging on earth compared to changes observed during space flight. On earth, the skeleton is continually loaded during normal activities. Load causes mechanical strains within the bone, which tend to be greatest on the subperiosteal surface, the connective tissue with bone forming cells attached to the surface (cortex) of the bone. In response, more new bone mass forms on the cortex. Simultaneously, the normal turnover of bone accompanying the aging process causes some net loss of bone from endocortical (inside the cortex) and internal surfaces. In long bones, the net loss under loading causes skeletal strains to increase most on the subperiosteal surface, not at the internal surfaces where the bone loss occurred. Because it takes less new bone on the subperiosteal surface to compensate for bone loss from internal surfaces, strength can be maintained in the presence of net bone loss.
During space flight, loading is practically absent on the lower skeleton. Not only does bone loss accelerate under diminishing loading, but evidence from cosmonaut data suggests that the compensatory changes are absent as well. This means that astronauts may be at a greater risk of fracture for the same loss of bone mass. Therefore it is important not only to determine bone mass, but also to determine the geometrical configuration of the bone structure. Bones loss countermeasures can be developed to increase the loading on the lower skeleton. The efficacy of such countermeasures is better determined individually, based on the geometrical configuration of the individual""s bone structure before and after the countermeasures, than by analyzing bone breakage statistics over a large population of astronauts. There is simply not a large population of astronauts.
Furthermore, the determination of bone structure is useful for screening a population and monitoring treatments of osteoporosis in postmenopausal women, elderly men and other susceptible individuals.
Loading and bone loss countermeasures can also be assessed through the measurements of muscle mass in a living human. Therefore it an advantage for a scanning device to also distinguish fat from muscle in soft tissue. Soft tissue excludes bone tissue.
There are several methods for determining bone mineral density (BMD), bone structure, and soft tissue components. These methods include computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and dual-energy x-ray absorptiometry (DXA).
While a CT unit can image and measure the geometrical characteristics of bone and soft tissue, it is not well suited for use in space because of its high radiation dose per scan. In addition, a CT unit capable of performing total body scans is extremely massive, weighing thousands of pounds. This great weight renders such units impractical for portable and space flight use. In addition, the high cost and large size place such units beyond the reach of small earthbound clinics, which might otherwise administer osteoporosis screening and treatment monitoring. An MRI unit is excellent for imaging soft tissues, for example to distinguish fat from muscle. However, an MRI unit suffers from a similar size and weight disadvantage. An MRI unit capable of performing whole body scans consumes significant power, generates large magnetic fields, and weighs tens of thousands of pounds.
Commercial scanners use dual-energy x-ray absorptiometry (DXA) or ultrasound to yield measurements of bone mineral density (BMD) that are regional averages. However, regional averages obscure structural details, and thus are not precise enough to deduce bone strength. Such systems do not predict risk of breakage. Furthermore, ultrasound devices have not been used successfully for the quantification of muscle mass.
In addition, commercial DXA devices consume too much energy for portable use. Furthermore DXA scanners employ ionizing radiation, which can pose a radiation risk to astronauts confined to operate in small spaces in the vicinity of a DXA device.
Based on the foregoing description, there is a clear need for techniques to derive bone structure, fat tissue mass and lean tissue mass from multiple projection, DXA systems.
Furthermore, there is a need for a system that yields a risk of injury including bone breakage.
According to one aspect of the invention, techniques for calibrating bone properties from dual-energy x-ray absorptiometry images, include receiving first image data and second image data. The first image data has pixels indicating attenuation through multiple known thicknesses of a first two calibration materials at a first photon energy. The second image data has pixels indicating attenuation through the multiple known thicknesses of the first two calibration materials at a second photon energy. A first conic-surface function and a second conic-surface function are determined. The first conic-surface function relates attenuation data from the first image data to the plurality of known thicknesses. The second conic-surface function relates attenuation data from the second image data to the plurality of known thicknesses. The first and second conic-surface functions are inverted to determine a pair of thickness functions. Each thickness function relates thickness of one calibration material to attenuations from the first image data and the second image data. The pair of thickness functions are applied with attenuations from image data comprising pixels indicating attenuation through tissue of a subject.
According to another aspect of the invention, techniques are provided for deriving bone properties from images generated by a dual-energy x-ray absorptiometry apparatus. The apparatus has an x-ray source in fixed relation to an x-ray receiver. The source and the receiver are moveably mounted to measure attenuation through a subject at multiple projection angles. First image data and second image data are received. The first image data has pixels indicating bone mineral density projected at a first angle of the multiple projection angles. The second image data has pixels indicating bone mineral density projected at a different second angle. Based on the first image data and the second image data, a magnification factor is computed. The magnification factor relates distances associated with pixels in the first and second image data to corresponding distances at a bone in the subject.
According to another aspect of the invention, techniques for deriving bone properties from images generated by the dual-energy x-ray absorptiometry apparatus includes receiving first, second, third and fourth image data. The first image data has pixels indicating attenuation of a first photon energy projected at a first angle. The second image data has pixels indicating attenuation of a second photon energy projected at the first angle. The third image data has pixels indicating attenuation of the first photon energy projected at a different second angle. The fourth image data has pixels indicating attenuation of the second photon energy projected at the second angle. A value indicating a bone mineral density for a particular pixel in the first image data is computed based on the first, the second, the third and the fourth image data.
According to an embodiment of this aspect, the step of computing the value indicating the bone mineral density includes determining a soft tissue composition for the particular pixel based on a set of pixels without measurable bone mineral density at the second angle. The value of the bone mineral density is determined based on the attenuation at the particular pixel in the first image data, the attenuation at a corresponding pixel in the second image data, and the soft tissue composition for the particular pixel.
According to another aspect of the invention, techniques for deriving bone properties from images generated by a dual-energy x-ray absorptiometry apparatus includes receiving image data having pixels indicating bone mineral density projected at a first angle. A long axis for a bone in the image data is determined. A first set of pixels is selected in the image data. The first set of pixels lie substantially along a line segment crossing the bone and perpendicular to the long axis. A cross sectional moment of inertia is computed for the bone based on the first set of pixels.
According to another aspect of the invention, techniques for deriving bone properties from images generated by a dual-energy x-ray absorptiometry apparatus includes receiving first image data, second image data and third image data. The first image data includes pixels indicating bone mineral density projected at a first angle. The second image data includes pixels indicating bone mineral density projected at a different second angle. The third image data includes pixels indicating bone mineral density projected at a third angle different from the first angle and the second angle. Principal moments of inertia for a bone in the subject are computed based on the first, the second and the third image data.
According to another aspect of the invention, techniques for deriving bone properties from images generated by a dual-energy x-ray absorptiometry apparatus includes receiving data indicating multiple principal moments of inertia for a bone in the subject. The data is based on images taken at several projection angles. A stress on the bone associated with a particular scenario is determined. A probability of the particular scenario is determined. A risk of injury is determined based on the probability of the particular scenario, the stress associated with the particular scenario, and the moments of inertia. The risk of injury is reported to a human operator.
These techniques allow derivations of bone and muscle strength, risk of injury, and efficacy of countermeasures among other properties, from high-precision bone structure measurements.